1. Field of the Invention
The present invention generally relates to a medical image forming apparatus for forming images in a region of interest from medical images of a computer tomography (CT) apparatus, a magnetic resonance imaging (MRI) apparatus, an X-ray photographing apparatus or the like. More particularly, the present invention relates to a medical image forming apparatus which is capable of forming definite images in a region of interest from ones tangled with bones and blood vessels in a human head or the like.
2. Discussion of the Background
In an X-ray CT apparatus which irradiates X-rays toward an examined body and photographs a tomographic image thereof, a preliminary scanning operation (called hereinafter "a pre-scan") may be carried out to judge whether or not a contrast medium reaches a photographing object portion (blood vessels, etc.). Such an X-ray apparatus is disclosed in U.S. Pat. Nos. 5,459,769 entitled "Procedure for Monitoring Contrast Agent Application in a CT Imaging System" issued to Barry D. Brown on Oct. 17, 1995 and U.S. Pat. No. 5,612,985 entitled "Computer Tomography Apparatus" issued to Yusuke Toki et al. on Mar. 11, 1997.
A series of operations is briefly shown in a flow chart: of FIG. 1. After a start operation 10 the X-ray CT apparatus executes a pre-scan preparation step 20. In this step, the apparatus is initialized and a predetermined level of X-ray exposure is set therein. Images to be scanned are photographer in response to the X-ray irradiation from the apparatus toward a subject body. The images are displayed on a monitor unit. An operator observes them carefully and determines an image at an appropriate place as a reference image Alternatively, if such images have been already stored in a recording unit through a previously diagnosed operation, the operator may select from them a CT image at a place suitable for a diagnostic purpose as the reference image. While observing the reference image, he or she manipulates a keyboard to set up a region-of-interest (called hereinafter "a ROI") in the reference image displayed on the monitor unit. This ROI is set to surround contrast medium observing regions with a circular frame, for instance. After determination of the reference image, a contrast medium is injected into a blood vessel of the subject body. The pre-scan 30 is then performed.
In this case, the operator checks the contrast medium by observing images in the ROI at a step 40 and instructs the X-ray CT apparatus at a start of an ordinary scan 50 to photograph CT images when he or she confirms that the medium has fully reached diagnostic portions. Thus, CT images useful for diagnosis are obtained because photographs thereof can be taken in such a preferable situation that the contrast medium has completely come to the diagnostic portions. The X-ray CT apparatus then ends its operations at a step 60.
Such confirmation of the contrast medium by means of the pre-scan is relatively easy in the case that a ROI is set up at rather larger blood vessels, e.g., abdominal aortas. However, confirmation is quite difficult for head portions, e.g., where bones and blood vessels are complicatedly located.
In the case of such head portions including complicated bones and blood vessel s, a CT value indicative of an X-ray absorption rate for each substance is almost the same and images set up in the ROI are gray in color as a whole. Fine blood vessels in the vicinity of bones, such as internal carotid arteries, are hardly displayed as images in the ROI. Thus, it is very difficult to check the contrast medium.